Distribution, morphology, and gamma-aminobutyric acid immunoreactivity of horizontally projecting neurons in the macaque inferior temporal cortex.

In area TE of the macaque inferior temporal cortex, horizontal axons running parallel to the pial surface mediate interactions between laterally displaced sites across the cortex. We examined the spatial distribution and the types of cells that give rise to these horizontal axons, which are important factors in determining the nature of the lateral interactions in TE. Intracortical injections of retrograde tracers labeled columnar clusters of cells and cells diffusely scattered within TE. The clusters were 0.35 +/- 0.11 mm (mean +/- SD) in diameter and were laterally distributed up to 6 mm from the injection site. Labeled cells were found in layers 2 to 6, with only a few labeled cells seen in layer 4. The clustering of labeled cells in layers 5 and 6 was looser than that in layers 2 and 3. Intracellular staining of the retrogradely labeled cells revealed that the majority of them were typical or modified pyramidal cells, both within and between the clusters. Only a few nonpyramidal interneurons were also stained at the fringe of the tracer injection site. Consistent with these results, only a small proportion of the retrogradely labeled cells exhibited gamma-aminobutyric acid (GABA)-like immunoreactivity, mostly found within 1 mm from the injection site. The results indicate that direct horizontal interactions in TE are predominantly mediated by pyramidal or modified pyramidal cells in layers 2, 3, 5, and 6 and are primarily excitatory in nature. The contribution of GABAergic interneurons to direct horizontal interactions is restricted to only short-distance projections.